Oracle's own documentation suggests that developers should not rely on this behaviour since it may change in the future.
Note:Oracle Database currently treats a character value with a length of zero as null. However, this may not continue to be true in future releases, and Oracle recommends that you do not treat empty strings the same as nulls.
pgsql-performance by date:
Соглашаюсь с условиями обработки персональных данных